Ok this is the procedure on my 02 WRX but it should be the same or very similar.
You have to take the bottom part of the back seat off first, there are two 12mm on the back part of the foot well. Just follow the front of the back seat down. Then the bottom part really just slides off.
There will be two more under where the bottom is holding the seat back on. Take those out, and make sure the pull out isn't latched in place, then the seat back slides up and off. There is a little bar that slides into a notch when you re assemble it.